1. Field of the Invention
The present invention relates to a method for producing a laser beam in micro spot and with deep depth of focus, and to an apparatus using the method.
2. Related Background Art
Conventional laser beams were generally Gaussian beams, which had restrictions based on propagation properties thereof. Recently, Bessel beams (J.sub.0 beams or nondiffracting beams) attract attention as laser beam having a very deep depth of focus and a relatively small diameter of spot. The details about Bessel beam are described for example in Durnin: J. Opt. Soc. Am. A, vol. 4, No. 4, p. 651 (1987). The Bessel beams have a feature that the light amplitude distribution is proportional to the zero-order Bessel function of the first kind in the cross section normal to the propagation direction. In detail, the amplitude distribution U(r) of Bessel beam may be expressed by the following equation with r being a distance to the optical axis. EQU U(r)=AJ.sub.0 (.alpha.r) (1)
In the above equation A and .alpha. are constants.
There are known methods for approximately obtaining a Bessel beam, for example, a method for forming a Bessel beam by using narrow ring apertures and a lens, which was proposed by Durnin et al. (Phys. Rev. Letters, vol. 58, No. 15, p. 1499 (1987)), and a method for forming a Bessel beam by using a conical prism, which was proposed by Kawada and Arimoto (Extended Abstracts p. 829, 30p-A-4 (Spring Meeting, 1991); The Japan Society of Applied Physics and Related Societies).
As apparent from the behavior of J.sub.0 (r), the Bessel beams, however, have difficulty in applications in that the intensity of side lobes of diffraction rings is high. For example, as shown in FIGS. 7A through 7C described below, the intensity of first ring is 16% of that of center peak and the intensity of second ring is 9% thereof. The existence of such submaxima (side lobes) made it difficult for Bessel beams to be applied in actual apparatus, for example in recording apparatus.